The semiconductor manufacturing industry is based upon very precise manufacturing processes as the scale of semiconductor devices continues to shrink. For example, integrated devices, vias, and other electrical pathways are being formed at smaller dimensions in accordance with performance and size constraints. Highly precise inspection systems are used to control the quality of today's semiconductor manufacturing systems. For example, the ability of inspection systems to detect defects on a specimen during a manufacturing process allows process engineers to fine tune a manufacturing system for greater yield and shorten the time needed to being a new facility or piece of processing equipment up to full production capacity.
Typically, an inspection system is interested in detecting defects that occur during the most recently formed layer or process of a semiconductor wafer. This is usually the top-most layer of the thin-film stack. Most of the current inspection systems use autofocusing techniques that track a signal from the brightest layer within a semiconductor wafer thin-film stack. Unfortunately, the top layer of inspected thin-film stacks can be transmissive and reflects little light, while underlying layers (such as Copper, Silicon, Aluminum, and Tungsten layers) reflect a significantly higher percentage of light giving the autofocus system the highest signal to operate on. This causes all inspection systems to determine best focus at the underlying layers while the top-most layer, which is the layer of interest for defect inspection systems, is left out of focus; unless a focus offset is applied. Even with an offset, all inspection systems will track the signal from the brightest layers as it varies up and down within a thin-film stack as the inspection system is scanned across a wafer. Thus, the top surface of a wafer may be out of focus by varying degrees while an inspection system scans across a wafer. Errors in proper focus position can be on the order of several microns in some cases, which is much larger than the depth of focus of many advanced wafer inspection and review tools.
Focus errors affect image contrast levels such that signals from defects will be weakened. This lowers the sensitivity of many inspection systems today. Furthermore, as the operational wavelengths of inspection tools shortened, the depth of focus of such inspection stools also shrink. This only exacerbates the problem of receiving weak signals from defects due to inspection systems that are not properly focused upon the top-most layer of wafer thin-film stacks. Therefore, out-of-focus inspection systems will have even worse sensitivity due to the smaller wavelengths, which were used initially to obtain higher resolution and sensitivity.
Some autofocus techniques utilize confocal autofocus techniques that allow inspection systems to attempt to locate and focus upon the top-most layer of thin-film stacks. In confocal optical systems, the image of a pinhole light source is focused by the optical system onto a point in the specimen and that point is again focused by the optical system onto a conjugate pinhole aperture with a detector located behind this pinhole to detect the signal. Confocal imaging systems provide signals that are sharply peaked about the conjugate focal points of pinholes in a spatially-coherent imaging system. This allows confocal imaging systems to accurately identify the depth of various surfaces within a thin-film stack by recording when they have high signals indicating a reflective surface at that focus location. In contrast, brightfield inspection systems uniformly illuminate a specimen and do not have apertures at the detector. Therefore, depth-signal curves for brightfield systems are flat and signal peaks for each layer of a thin-film stack are not identifiable. However, one issue with confocal imaging is that it provides a signal only where the conjugate images are sampling the depth. Unfortunately, signal intensity drops off quickly at points above and below the focal points because of the limited light collection at the detector for out-of-focus locations. Thus, they only provide limited depth and spatial information unless scanned in some manner.
Some attempts to mitigate this issue in order to inspect a larger area of a specimen involve modulating or scanning an aperture's focus location along the depth a thin-film stack. These techniques involve adjusting the height of an aperture formed in a mask, which is placed in front of an illumination source, while the inspection system is scanned across the surface of a specimen. However, this proved impractical as the layers within the thin-film stack are only partially sampled because the height of the focus location cannot be scanned throughout the entire depth of a thin-film stack at a particular location before the field of view of the inspection system moves past that particular location. Also, the confocal scanning data is blurred due to the motion of the stage for the inspection or review system. Furthermore, a signal for a layer might disappear completely if that layer is etched away in certain locations, or due to thin-film interference that just happens to return a destructive interference condition yielding no reflected signal. Unfortunately as a result, these concepts provide only partial (or sampled) depth information regarding the layers of a thin-film stack. Therefore, these techniques are unreliable for locating and focusing upon a top surface of a thin-film stack.
In view of the foregoing, there are continuing efforts to provide an improved autofocus subsystem that locates a specific layer or offset to a particular layer of interest, such as a top-most layer, within a semiconductor wafer or lithography-reticle surface inspection or review system would be desirable.